Coordinated Multi-Point (CoMP) transmission/reception has been proposed as a promising technology to meet the 3GPP (Third Generation Partnership Project) LTE-Advanced (LTE-A) requirements by improving performance of cell-edge UEs in particular. In CoMP operation, multiple transmission/reception points (typically geographically separated, but could also be co-located) cooperatively transmit to or receive from one or more users' equipment (UEs) to improve performance, especially the performance of cell-edge UEs. In the case of downlink CoMP, each transmission point, which can have one or more transmit antennas, is a radio unit whose signal covers a geographical area. Multiple coorperating CoMP transmission points may connect to a same base station (referred to as eNodeB or eNB in LTE) or may belong to multiple eNBs with each eNB connecting to one or more transmission points. Further, each transmission point may form its own cell or multiple transmission points may form a single logical “cell.” From a user equipment (UE) perspective, a cell usually corresponds to an associated cell identifier (cell-ID), which is typically used to scramble the data and pilot signals (also referred to as reference signals) transmitted to UEs associated with that cell.
In conventional non-CoMP operation, a single transmission point, which is the serving cell of a UE, adapts the transmission parameters based on the quality of the link to the UE. In this so-called link adaption as commonly adopted in modern wireless communications, a UE needs to estimate a channel quality of a hypothetical data transmission which is traditionally from a single cell for non-CoMP operation. Channel quality is often represented as a modulation and coding scheme (MCS). UE may also feed back some recommendation of spatial transmission parameters, such as transmission rank indication, precoding matrix index, and the like. In CoMP operation, the transmission from multiple points also needs to adapt to the link condition as seen by the UE.
The UE relies on pilot signals (also known as reference signals or RS) sent from a serving cell for channel estimation (and subsequent data demodulation) and for channel quality measurements that are reported back to the eNB. Often the reference signals are scrambled with a sequence specific to a cell-ID of that particular serving cell. In order to estimate a channel and to make channel quality measurements, the eNB must have a mechanism that enables the UE to estimate the channel and also measure the interference. The usual mechanism to enable the channel estimation by the UE is for the eNB to send pilot signals from each of the transmit antennas, which essentially sound the channel. A pilot signal is a set of signals known by both the transmitter and receiver. In OFDMA systems, the pilot signals usually correspond to a time/frequency grid of resource elements (REs), where a resource element is a subcarrier in OFDM transmission. The UE would then use the pilot signals to compute channel estimates at each subcarrier location by performing interpolation and noise suppression, and measures a channel quality. Further pilot signals are also needed at the UE to reconstruct the “effective” channel for purpose of demodulation. An effective channel corresponding to one or more data streams of a UE is the precoded/beam-formed channel that a UE effectively sees applied to a data modulation signal at the receiver.
In Releases 8 and 9 of the 3GPP LTE standards, two different types of pilot signals, that is, RSs, are used for these purposes. Common or Cell-Specific reference signals (CRS) are sent from an eNB and are intended for all UEs in a cell served by the eNB. CRS could correspond to the set of physical antennas at an eNB or a set of virtualized antennas observable at all UEs. These RSs may be used for channel estimation for channel quality and spatial feedback measurements where a UE can compute a recommended antenna weights for maximizing performance at the UE. These RSs can also be used for demodulation, but an eNB has to let the UE know in some manner what antenna weights (also known as transmit precoding matrix indicator (PMI)) are used. The
UE can then construct the “effective” channel based on the measured channel and the known transmit PMI information.
On the other hand, Dedicated Reference Signals (DRSs), that is, user-specific pilot signals (also referred to as UE-specific RS), are intended for a particular UE only and, in a typical operation, are only present on resources allocated to that user. Further, they usually represent the effective channel, which can be directly used for demodulation. This gives the eNB the flexibility to use user-specific precoding or beamforming on these allocations transparently without explicitly indicating the precoding to the intended UEs.
Currently, interference measurements can be measured on these reference signals. For purpose of feedback, which is based on CRS in Release 8, interference can be measured directly on CRS after subtracting channel information from received signals. For purpose of demodulation, the interference typically is measured on DRS directly, if they are used for demodulation, or based on CRS if DRS is not present. Either way, both of these RSs, that is, CRS and DRS, are transmitted with sufficient density that allows good interference measurements.
In Release 10 of the 3GPP LTE standards, a new type of reference signal, namely a Channel State Information Reference Signal (CSI-RS), has been defined for performing channel estimation, mainly for feedback purposes. In other words, CSI-RS is not intended for use in demodulation (like CRS), but only for enabling channel state measurements at the UE for reporting feedback. This specialized requirement for CSI-RS implies that CSI-RS may be sent less frequent in time and frequency, as opposed to the larger density needed for demodulation purposes. As an example, CSI-RS may be sent with a spacing of 12 subcarriers (as opposed to three subcarriers with CRS) in the frequency domain and only as often as every five subframes (as opposed to every subframe for CRS).
However, interference measurements on CSI-RS are expected to be very inaccurate compared to CRS based measurement, due to the infrequent transmission of CSI-RS. Furthermore, in CoMP, since multiple cells or multiple transmission points (like remote radio units (RRUs)) may be jointly transmitting to a UE, one or more neighbor cells may be actively participating in interference reduction to a UE. As a result, interference measurements that are conventionally defined to be ‘any interference other than the serving cell’ are not always valid. That is, the set of cells that should be counted as interferers could depend on the set of participating CoMP transmission points. Interference measurements directly on CSI-RS may not always reflect the actual interference. Therefore, there is a need for additional definitions of configurations for interference measurements that address multi-cell CoMP operations.
In other words, a need exists for enabling both channel estimation and interference measurement at a UE in a 3GPP LTE-A communication system and that accounts for the CoMP interference correctly.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and/or relative positioning of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention. It will further be appreciated that certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required. Those skilled in the art will further recognize that references to specific implementation embodiments such as “circuitry” may equally be accomplished via replacement with software instruction executions either on general purpose computing apparatus (e.g., CPU) or specialized processing apparatus (e.g., DSP). It will also be understood that the terms and expressions used herein have the ordinary technical meaning as is accorded to such terms and expressions by persons skilled in the technical field as set forth above except where different specific meanings have otherwise been set forth herein.